1. Field of the Invention
The present invention relates to a method and apparatus for protecting and monitoring the transmission of information between the central unit of a programmable controller and the sensors and/or actuators of the process controlled by said controller.
2. Description of the Prior Art
Generally, it is known that a programmable controller uses a processor for the commanded control of the actuators of the process as a function of the information supplied by the detectors, according to an operating sequence determined by a programme.
Providing its connection with the process, the controller comprises a plurality of connecting elements broken down into two assemblies, namely:
an assembly of input connecting elements to which the detectors or sensors are connected, and
an assembly of output connecting elements to which the actuators of the process are connected.
The processor generally comprises a central unit, usually with microprocessor, in which the transfer of the information takes place through buses (data bus, address bus, control bus). This central unit communicates with the outside and in particular with the input/output connecting elements of the controller by means of input/output circuits connected to the buses generated by the microprocessor.
Generally, these input/output circuits are formed on one or more printed circuit boards each grouping together a predetermined number of input and/or output channels.
Each of these channels ends in a multipole input or output materialized by a pole connecting area on the printed circuit. It will be noted in this connection that very often bipolar inputs and outputs are used and, accessorily, three pole outputs (AC outputs).
The connecting areas may be advantageously disposed along one edge of the printed circuit, on which edge a connecting strip may be connected, for example of the type described in patent application published under No. 2 544 556 on Oct. 19, 1984 in the name of the applicant. This connecting strip is intended more particularly to provide a disconnectable connection between said areas and the corresponding conductors connecting the controller to the detectors and to the actuators of the process.
Thus, in a control of the kind mentioned above, the aim of the invention is first of all to provide input/output circuits in which the inputs and outputs of each of these channels are suitably isolated from the bus conductors of the central unit so as to protect the central unit against any disturbance occurring in the connecting strip or the external circuits (detectors, actuators), these disturbances consisting more particularly of overloads, short circuits or deficiencies in the external supply.
In addition, the invention has a further aim of adjoining, to the above mentioned input/output circuits, respective circuits for detecting transmission defects resulting more particularly in a non concordance between the signals transmitted and received by the bus of the central unit and the corresponding signals transmitted by the detectors or received by the actuators, this non concordance being more especially due:
to the non execution of an output order, for example because of overloads, short circuits or deficiencies in the external supply not perceived by the central unit;
to an input being forced to zero and to the non supply of an actuator, through disconnection of the terminal strip not perceived by the central unit;
to certain input/output channels being forced to zero or to one, through deficiencies of the interface circuits not perceived by the central unit.